1. Field of the Invention
This invention relates to an antenna and, particularly to a telescopic antenna having an outer conductor and an inner conductor.
2. Description of the Related Art
In some portable telephones telescopic antennas are used. FIGS. 1A and 1B show an example of the exterior appearance of such a portable telephone and telescopic antenna. In FIGS. 1A and 1B, a telescopic antenna 2 is attached to the top of the main body of a portable telephone 1 by way of a coaxial connector 3.
The telescopic antenna 2 shown in FIGS. 1A and 1B is of two-stage telescopic type. That is, when the portable telephone 1 is on standby the telescopic antenna 2 is in a retracted state as shown in FIG. 1A, and during calls the telescopic antenna 2 is in an extended state as shown in FIG. 1B. If the wavelength of the communications frequency used by the portable telephone 1 is written .lambda., the electrical length of the telescopic antenna 2 when retracted as shown in FIG. 1A is .lambda./4 and the electrical length of the telescopic antenna 2 when extended as shown in FIG. 1B is .lambda./2.
The portable telephone 1 can be made in a compact form, for example about 80 mm (height).times.55 mm (width).
However, because portable telephones use frequencies in the 900 MHz band for transmission and reception, the wavelength .lambda. is about 33 cm. As a result, the length of the telescopic antenna 2 when retracted is about 83 mm, and when the length of the connector 3 is included the length of the telescopic antenna 2 as a whole is about 87 mm.
Consequently, when the portable telephone 1 is on standby and the telescopic antenna 2 is retracted as shown in FIG. 1A, the balance between the height of the portable telephone 1 and the length of the telescopic antenna 2 is poor. Also, because the length of the telescopic antenna 2 when it is retracted is .lambda./4, this places restrictions on the design of the portable telephone 1. Similar problems arise when the telescopic antenna 2 is extended as shown in FIG. 1B.